This invention relates to an electronograph provided with a photosensor formed with non-crystalline silicon (a-Si:H) as a photoconductive material.
Known photoconductive materials used as photosensors for electronographs include selenium tellurium alloys, selenium arsenic alloys and the like made mainly by mixing together semiconductors such as zinc oxide (ZrO), cadmium sulphide (CdS), polyvinylcarbazol (PVK) and amorphous selenium (a-Se) and amorphous silicon (a-Si). Although each of these materials for use in photosensors has its own advantages, it has not always fully satisfied the performance requirements of the photosensor. Particularly with the recent appearance of high-speed copying machines or printers, demand for photosensors with excellent abrasion resistance is increasing.
A photosensor a-Si:H meets such requirements because of its superior mechanical properties and abrasion resistance. In addition, it has excellent heat resistance because its crystallization temperature is higher than that of a-Se. Furthermore, the use of a-Si:H produces less environmental pollution. Because of these benefits of a-Si:H, there has been intense development of this material for utilization in photosensitive layers for electronographs.
However, it has been discovered by the present inventors and others that the charge-storage capacity of a photosensitive layer containing a-Si:H varies to a large extent with the type of radiation that is used. Layer fatigue is increased by repeated irradiation and this deteriorates the charge-storage capacity of such a layer and reduces its image quality. This layer fatigue results from the fact that some of the charge carriers produced by the irradiation remain in the photosensitive layer. In other words, rays of light incident to the surface of a photosensitive layer, excluding those reflected, penetrate into and are absorbed by the layer. Some of the carriers generated by the light are extinguished through recombination and some are trapped by defects in the layer and stay in the photosensitive layer. This reduces the electric potential in the layer because the release of the trapped carriers is accelerated when an electric field is applied during the subsequent charging process. The trapped carriers during this time are not locked in the carrier traps, but are released because of thermal excitation so that the number of trapped carriers decreases. Accordingly, the reduction of the electric potential resulting from irradiation depends upon the length of time after the irradiation, particularly after the application of discharging rays of light which are stronger than the image exposure. Thus, the reduction of the potential is theoretically decreased further if the length of time between the irradiations is increased.
It is, however, not practical to utilize a copying process that requires a significant time between the initial irradiation and the operation of the charging device.
An object of the present invention is to provide an electronograph with an a-Si:H photosensitive layer that is resistant to fatigue caused by light using means readily applicable to a commercially practical apparatus.
Other objects of the invention will become apparent from the description of the preferred embodiment or may be learned by practice of the disclosed invention.